This invention relates to power supplies for gas discharge lamps and in particular to high intensity gas discharge lamps such as metal halide lamps.
High wattage, high intensity metal halide lamps are desirable because of their natural color output and find application as a light source for fiber optic surgical inspection devices and stage lighting. One difficulty with high intensity metal halide lamps is that their lamp life is short, typically under 100 hours. In order to preserve lamp life, it is desirable to apply constant wattage to the lamp. This is a somewhat complicated task because the natural impedance of the lamp changes over its lifetime so that a constant current or a constant voltage source alone are not optimal for providing maximum lifetime.
Another characteristic of metal halide lamps is the very high voltage required to strike an arc in the lamp. Even utilizing a voltage multiplying circuit to establish an arc, the conventional power supply must supply such a striker circuit with a nominal 300 volts to establish the arc. The voltage from the power supply is multiplied by the striker circuit to typically 30,000 volts. Once the arc is struck, however, the lamp operates as a voltage regulator, establishing a 40 to 80 volt drop across its terminals and, therefore, requires a reduced voltage from the power supply. One common failure mode for a power supply for such a lamp is high voltage breakdown to chassis ground during the arc striking phase. An isolation transformer between the power supply and striker circuit will greatly reduce the likelihood of such a failure. However, an isolation transformer that is capable of providing the necessary 300 volt supply during striking, without saturating the transformer, is large and bulky compared with the size of the transformer required to operate the lamp at 40 to 80 volts after the arc has been struck.
An additional consideration in providing a power supply for a high intensity metal halide lamp is that, due to their relatively short lifetime, such lamps are typically switched off and on frequently to maximize their longevity. Conventional power supplies utilize an input thermistor to limit the in-rush current upon initial startup. The thermistor has a higher "cold" resistance value, which decreases as the thermistor heats up. If such a supply is switched off and back on again in a relatively short cycle, such as one second, the thermistor will not have had an opportunity to cool sufficiently to reestablish a high resistance. To handle such circumstance, a conventional supply is fused at a higher than desirable level.